A standard drilling apparatus of the type used to bore a plurality of holes in a workpiece, for instance in the web of a C- or I-beam, typically has a transverse support beam on which a drilling unit can move transversely of the workpiece, while this beam itself can move portal-fashion longitudinally of the workpiece. The support carries a vertically or horizontally displaceable arbor holding a drill bit that can move vertically or horizontally through a relatively long transverse stroke. The drill bit is rotated continuously and the actuator that moves the arbor and bit through this stroke normally advances the bit toward the workpiece and retracts it therefrom at a relatively fast speed, but only moves the bit at a relatively slow speed while the bit is actually boring.
As described in U.S. Pat. No. 4,108,564 the displacement of the arbor is typically controlled by a switch fixed relative to the arbor and operated by an actuator on the arbor. Thus the operator on the arbor can actuate the switch that controls the axial movement of the arbor and bit, for instance by restricting flow into or out of the actuating cylinder for this arbor.
An abutment and stop are provided that are operatively mounted on the arbor and drill bit to define the uppermost position of the arbor, that is its position furthest from the workpiece. The length of the stroke through which the drill bit and arbor are moved from this uppermost position to contact with the workpiece is normally somewhat long, so that the bit can be retracted enough to allow workpieces with fairly high parts to be moved under it.
When a succession of holes are to be bored in the workpiece, something which is fairly common when workpieces are being prepared for a prefabricated project, the drilling arbor is moved through its entire vertical stroke between successive boring operations. Even though the major portion of the stroke is carried out at relatively high speed this long up and down reciprocation takes up quite some time and generally slows the boring operation when such a succession of bores must be made.